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BLM system calibration:

Date post: 21-Jan-2016
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BLM system calibration: The present BLM system reports in Rads/sec in ACNet. If we know this comes from an instantaneous (< millisecond) loss, the nominal factor to go from Rads/sec at peak to Rads is the time constant, 60 milliseconds, - PowerPoint PPT Presentation
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1 BLM system calibration: resent BLM system reports in Rads/sec in ACNet. If we know comes from an instantaneous (< millisecond) loss, the nominal fact o from Rads/sec at peak to Rads is the time constant, 60 millisecon ads = Rads/sec x 0.06. The data to the left come from proton injections for the last store in August 2004. Loss monitor LMF12 has a peak loss of 0.025 rads/sec which implies a loss of 0.025 x 0.06 = 0.0015 rads or a charge of 105 pC; loss monitor LMF0L3 has a peak loss of 0.1 rads/sec which implies a loss of 0.1 x 0.06 = 0.006 rads or 0.42 nC - but see later Fig. 1
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Page 1: BLM system calibration:

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BLM system calibration: The present BLM system reports in Rads/sec in ACNet. If we know this comes from an instantaneous (< millisecond) loss, the nominal factor to go from Rads/sec at peak to Rads is the time constant, 60 milliseconds, so Rads = Rads/sec x 0.06.

The data to the left come fromproton injections for the last store in August 2004.Loss monitor LMF12 hasa peak loss of 0.025 rads/secwhich implies a loss of 0.025 x 0.06 = 0.0015 rads ora charge of 105 pC;loss monitor LMF0L3 has a peak loss of 0.1 rads/secwhich implies a loss of0.1 x 0.06 = 0.006 rads or 0.42 nC - but see later

Fig. 1

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This shows the same lossmonitors LMF0L3 and LMF12read out by a new digitizerboard. The 2nd and 4th tracedown are the respective rawdata; the 1st and 3rd traces are smoothed over 5 samples.

For the new digitizer,65,536 counts = 10V x 100 pF=> 1 count = 0.015 pCLMF12 (2nd trace from bottom)has 300 x 5 = 1500 counts=> q total = 22.5 pC(new system)Per R. Shafer, 1 rad = 70 nC,=> 0.0015 rads = 105 pC(present system)

so I am low by a factor of 5

??????? Help ! ! ! ! ! ! ! !

0.5 milliseconds/box

LMF0L3

LMF12

Fig. 2

850 peak

1100 peak

300 peak

400 peak

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The conversion used by ACNet to go from volts in the MADC to Rads/sec

is: Rads/sec = 0.011 x 10 ^ (V/2.39)

So a reading of 0.025 Rads/sec => V = 2.39 x log10(0.025/0.011) Volts

= 2.39 x 0.36 Volts

= 0.86 Volts

The plots on the next page show the actual output voltage vs the input

current: the offset bias current has a significant effect below 2 Volts - the

actual input from the BLM is more like 60 pC.. this helps to reconcile

things.

As important as this effect, however, is that the losses actually last a long

time - and the estimate made by looking at the fast peak is rather

misleading. The continuing losses are evident looking at the signal from

LMF0L3 as seen in the new system in figure 2. The signal clearly does

not return to its baseline after the initial spike. Figure 5 shows the integral

of the LMF0L3 signal - a good 2/3 comes after the initial spike.

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Blue points (and line) are D80 conversionand assume1 rad/sec = 70 nA

amps

coulombs

0.1 r/s

1 r/s

Fig. 3Fig. 4

Page 5: BLM system calibration:

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The red is the lossof LMF0L3; the blue is theintegral of the LMF0L3 loss(note the factor of 100in the Scale/ box).

The baseline for the integral calculation is estimated using 833(=50 kHz/60) samples from thebeginning of the sampling.

The loss lasts for 25 milliseconds during which we accumulate (700 - 250) *100 = 45,000counts, 2/3 of themafter the initial spike.

45,000 counts = 0.75 nC

25 milliseconds

Integral(multiply scale by 100)

Sliding Average of 5 samples

Fig. 5

peak = 800

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If I look more carefully (ie with present knowledge) at the loss plot of the present BLM, a better estimate of the total loss is the peak x 100 milliseconds.This would imply a total loss of 0.01 Rads or a charge out of the BLM of 0.7 nC to be compared with the 0.75 nC from the new system.The closeness of the two numbers is satisfactory - and I have learnt at least three things.

1) We need to treat low losses reported by the present system a little carefully.

2) It is hard with the present system to distinguish losses that last tens of milliseconds from instantaneous losses - obvious - and these losses show such behavior.

3) The digitizer scale is (at least roughly) correct and we can apply our mindsto deciding the proper range. At present the biggest amount of charge we can take in one sample is 1 nC which corresponds to an instantaneous loss of 0.015 Rads; the largest continuous current we can take is 50 A which corresponds to a loss of 700 Rads/sec. The latter is much larger than the present system - good; the former is much smaller than the present system - possibly bad.

Note that instantaneous in the new system means measured over 20 microseconds; for the present system instantaneous means less than 30 or so milliseconds. The new system as presently arranged can deal with 0.75 Rads in one millisecond.

The next page shows the loss rates presently set for aborting in the Tevatron.

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Abort limits in the Tevatron are set at ~10 Rads/sec which implies 0.6 rads instantaneous loss.

If this loss is actually over one turn, this is 40 times bigger than anything the new system can measure.If it occurs over 1 millisecond or more, the new system can just cope.

This has prompted us to consider ways to increase the instantaneous loss capability of the system; see BeamsDoc 1417


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